1. Field of the Invention
This invention relates to a catadioptric reduction projection optical system suitable for application as an optical system for reduction-projecting a pattern more enlarged than the pattern of a real element which is used, for example, in an exposure apparatus for the manufacture of semiconductive elements.
2. Related Background Art
Semiconductive integrated circuits have become more and more minute and higher resolving power is required of exposure apparatuses for printing the patterns thereof. To satisfy this requirement, the wavelength of a light source must be shortened and the numerical aperture (N.A.) of an optical system must be made great. However, if the wavelength becomes short, glass materials which stand practical use for the absorption of light become limited. If the wavelength becomes 300 nm or less, practically usable glass materials will be only synthetic quartz and fluorite (calcium fluoride). Fluorite is bad in temperature characteristic and cannot be used in a great quantity. Therefore it is very difficult to make a projection lens of a refracting system alone. Further, due to the difficulty of aberration correction, it is also difficult to make a projection optical system of great numerical aperture of a reflecting system alone.
So, various techniques of combining a reflecting system and a refracting system to thereby construct a projection optical system have heretofore been proposed. An example of them is a ring field optical system as disclosed in U.S. Pat. No. 4,747,678 (corresponding to Japanese Laid-Open Patent Application No. 63-163319). In this optical system, an off-axis light beam is used so that incident light and reflected light may not interfere with each other, and the design is such that only the off-axis zonal portion is exposed.
As another example, a projection exposure apparatus comprising a catadioptric system in which a beam splitter is disposed in a projection optical system, whereby the image of a reticle (mask) is collectively projected by an on-axis light beam is disclosed, for example, in U.S. Pat. No. 3,698,808 (corresponding to Japanese Patent Publication No. 51-27116) and U.S. Pat. No. 4,953,960 (corresponding to Japanese Laid-Open Patent Application No. 2-66510).
FIG. 3 of the accompanying drawings schematically shows the optical system disclosed in U.S. Pat. No. 4,953,960. In FIG. 3, a light beam from a reticle 21 on which a pattern to be reduction-transferred is depicted is converted into a substantially parallel light beam by a lens unit 22 having positive refractive power and is applied to a prism type beam splitter (beam splitter cube) 23. The light beam transmitted through the joint surface 23a of the beam splitter 23 is diffused by a correcting lens unit 24 having negative refractive power and is reflected by a concave reflecting mirror 25. The light beam reflected by the concave reflecting mirror 25 passes again through the correcting lens unit 24 and is reflected by the joint surface 23a of the beam splitter 23, whereafter it is converged on a wafer 27 by a lens unit 26 having positive refractive power, and the reduced image of the reticle pattern is formed on the wafer 27. An example in which a semi-transparent mirror comprising a plane parallel plate is used instead of the prism type beam splitter is also disclosed in the aforementioned patent.
In the ring field optical system of the prior art, however, it is difficult to make the numerical aperture great. Moreover, exposure cannot be collectively effected and therefore, it is necessary to effect exposure while moving the reticle and the wafer at different speeds correspondingly to the reduction ratio of the optical system, and this has led to the inconvenience that the construction of a mechanical system becomes complicated.
Also, the construction disclosed in the aforementioned U.S. Pat. No. 3,698,808 suffers from the inconvenience that the flare by the reflection on the refracting surface of the optical system subsequent to the beam splitter is great. Further, no consideration is given to characteristics such as the reflectance irregularity, absorption and phase variation of the beam splitter and therefore, the resolving power is low and also, the magnification of the entire system is one-to-one magnification, and the apparatus of this patent cannot possibly stand the use as the exposure apparatus for the manufacture of semiconductors of the coming generation of which higher resolving power will be required.
Further, of the projection optical system disclosed in U.S. Pat. No. 4,953,960 the optical system of FIG. 4 thereof suffers from the inconvenience that the resolving power is deteriorated by the non-uniformity of the material of the bulky prism for the beam splitter 23. It also suffers from the inconvenience that there is no adhesive agent usable for the wavelength range of about 300 nm or less and it is difficult to construct a beam splitter by cementing two blocks together. Also, that projection optical system has suffered from the inconvenience that a stop is at a location overlapping with the beam splitter 23 or the semi-transparent mirror and the stop cannot be physically placed. Thus, the resolving power is deteriorated, the irregularity of the quantity of light cannot be corrected and further, the telecentricity of the wafer 7 side cannot be secured, and this apparatus has not been practical as a semiconductor exposure apparatus.